The invention relates to a diffraction grating with a multiplicity of parallel diffraction structures succeeding one another periodically, which are arranged on a support and each incorporate a slat extending from a base area of the support. The invention also relates to uses for such a diffraction grating.
Known diffraction gratings of this kind exhibit in particular in the polarisation direction, which lies normal to the longitudinal extension direction of the slats (TM polarisation), a relatively poor reflection efficiency, i.e. a relatively poor diffraction efficiency in the order of diffraction employed. The latter furthermore fluctuates greatly with the angle of incidence or the light wavelength. The breadth of variation of the reflection efficiency in a relatively narrow range around the angle of incidence or the light wavelength, i.e. the distance between a minimum and a maximum value within said range, lies in the range of multiples of ten per cent.
If fixed light wavelength and angle of incidence values are set in advance, by virtue of the optical arrangement in which the known diffraction grating is used, the reflection efficiency obtained to date has been left to chance: e.g. there may be present in the TM polarisation direction for said parameters a bare minimum of the reflection efficiency. This cannot be tolerated in applications in which a high reflection efficiency of the diffraction grating with a pre-set light wavelength and a pre-set angle of incidence is required, e.g. a reflection efficiency in the range of the maximum value of the reflection efficiencies with said polarisation direction which are achievable for said type of grating in the area around the pre-set incidence direction or light wavelength.
It is therefore the object of the present invention to develop a diffraction grating of the kind given in the introduction in such a way that as high an efficiency of the grating as possible is achieved.
Said object is achieved according to the invention by the fact that the slat exhibits a substantially rectangular cross-section, wherein the width of the slat, measured normal to the centre plane of the slat and parallel with the base area, comes to less than 100 nm, preferably between 20 nm and 60 nm, more preferably in the area of 50 nm.
The slats of the diffraction grating according to the invention with said slat widths are also referred to below as xe2x80x9cnarrowxe2x80x9d slats. It may be shown by a calculation which incorporates the interaction between the light beams with TM polarisation and the slats that, surprisingly, a considerably better reflection efficiency of the diffraction grating is obtained with said slat widths, including for TM polarisation. The latter moreover, depending on the incidence direction and the wavelength of the incident light beams, does not vary strongly as in the case of known diffraction gratings with wider slats. Gratings with said narrow slats may accordingly be adapted, namely to the particular application in which angle of incidence and light wavelength are pre-set, by the choice of a suitable grating constant, without the above variations in the reflection efficiency for TM polarisation having to be allowed for or a special optimisation of the diffraction grating to the pre-set parameter pair xe2x80x9clight wavelengthxe2x80x9d and xe2x80x9cincidence directionxe2x80x9d having to be carried out.
Although the best reflection efficiencies are obtained from the calculation for slat widths in the area of 10 nm, a slat width of 50 nm appears to be the best compromise between reflection efficiency and cost of manufacture.
The height of the slats above the base area may come to more than 200 nm, preferably between 200 nm and 600 nm. It was likewise found, surprisingly, in the calculation incorporating the polarisation interaction, that the reflection efficiencies depend on the slat heights. Above a slat height of 200 nm the reflection efficiencies moreover approximated very rapidly to an optimal value which recurs only periodically on further increasing of the slat height. There is obtained here, once again on manufacturing grounds, the lowest slat height pertaining to a given reflection efficiency.
The centre plane of the slat may lie normal to the base area. A diffraction grating of this kind may be manufactured relatively simply, since the privileged direction of the slat working lies normal to the base area of the support.
Alternatively the centre plane of the slat may include with the base area an angle other than 90xc2x0 . By means of such an inclination of the slats compared with the base area of the support it may be additionally achieved that the slat flank facing the incident light beam lies in the area of a blaze angle for the particular application of the diffraction grating. An additional increase in the reflection efficiency is the result.
Preferably the diffraction grating consists of quartz glass or silicon. Such materials may be worked by reactive ion beam etching (RIBE) or by reactive ion etching (RIE) and are therefore considered for the holographic production of the diffraction structures according to the invention. If crystalline material such as silicon is used, it is possible in addition to align the crystal surface for the crystallographic orientation of the crystal in such a way that a privileged direction for the working, e.g. by anisotropic chemical etching with KOH, is obtained. Said privileged direction may be exploited e.g. in the production of inclined slate according to claim 4.
Alternatively the diffraction grating may consist of doped quartz. Such a material is suitable to be used because of its advantageous expansion characteristics.
If the diffraction grating consists, according to an alternative embodiment, of a dielectric layer system, the layer sequence may be so selected that an additional coating increasing the reflection efficiency of the diffraction grating may be dispensed with.
The layer system may furthermore comprise a multiplicity of mutually succeeding layers of Al2O3 (high refractive index) and MgF2 (low refractive index) or of LaF3 (high refractive index) and MgF2 (low refractive index). An alternating layer system of this kind is highly suitable for the production of a highly reflecting layer for ultraviolet light wavelengths.
If the diffraction grating comprises a coating increasing the reflectivity, the execution of the support as a dielectric layer system may in turn be dispensed with, wherein a high reflectivity is nevertheless achieved. In the case of optical applications which permit a subsequent coating with a reflection layer, a more moderately priced manufacture of the diffraction grating is achieved in this way.
Preferred as a coating is an aluminium coating. A coating of this kind is relatively inexpensive and exhibits a high achievable reflectivity.
It is a further object of the present invention to indicate uses for the diffraction grating according to the invention in which the described advantages of the diffraction grating are put to good use. Said object is achieved by the preferred uses listed below:
A preferred use of the diffraction grating is that as a reflection grating in a Littrow configuration. In such an optical arrangement, with a given, often production-related grating constant and a fixed light wavelength, the incidence direction of the light beams is also fixed. There is no margin of variation, therefore, in the layout of the optical arrangement, so that full use is made of the advantage that the reflection efficiency of the diffraction grating with TM polarisation does not depend critically on the parameters xe2x80x9cincidence directionxe2x80x9d and xe2x80x9clight wavelengthxe2x80x9d.
Preferably the diffraction grating is used in third order of the light wavelength. Particularly with small light wavelengths, e.g. in the UV range, reduced requirements of the production of the diffraction grating are thereby obtained, since in third order a greater spacing of the diffraction gratings than in the first order meets the diffraction condition.
An advantageous field of use of the diffraction grating is the diffraction of UV light with a wavelength which is less than 250 nm. Such UV light sources are used in particular in projection lithography for the production of semiconductor structures in chip manufacture. With a diffraction grating according to the invention a wavelength which is narrow-band and efficient may be selected for such a projection light source.
A further preferred use of the diffraction grating is as a phase grating during interferometric length measurement. With said use a high grating efficiency is likewise required, which is ensured by the use of the diffraction grating according to the invention.